1. Field of the Invention
The present invention is generally directed to a method and system for the downhole separation and injection of water contained in produced mixtures from a production zone of a hydrocarbon well.
2. Background
In many hydrocarbon wells, there is a high percentage of water (referred to as water cut) in the produced fluid mixture. In typical practice, the produced fluid is lifted to the surface and the water is separated from hydrocarbon at the surface. Surface separated water is subsequently treated and disposed of on the surface or re-injected into a subterranean formation for disposal or as part of an enhanced reservoir recovery program. This process is not always entirely satisfactory because of the energy needed to lift the water to surface, and costs involved in separation of the water and hydrocarbon fluid, and re-injection of the water.
In many cases, it might be more economical to separate the water downhole in the same wellbore and re-inject it into a suitable zone accessible through the same wellbore. Examples of methods for the downhole separation and re-injection of water contained in fluids produced from hydrocarbon wells have been described in the patent literature: WO86/03143, U.S. Pat. Nos. 4,805,697, 5,296,153, 5,456,837, 5,711,374, and 5,730,871. These approaches describe various means to achieve downhole separation of oil and water components of produced fluids with subsequent lifting of separated oil to the wellhead. These approaches rely on downhole pumps to re-inject the water component into a suitable zone, and to bring the oil to surface.
In order to drive a downhole pump, some form of power, be it mechanical, electrical or hydraulic, must be transmitted from surface to the pump. Hydrocarbon wells are often located in places where providing for power for such functions is not convenient.
In offshore wells, gas-lift systems are often preferred due to the simplicity and reliability of their associated downhole components. In such techniques, compressed gas is commingled downhole with the produced fluids, thereby reducing the density of the produced fluids until the weight of the column of the gasified fluids becomes less than the pressure exerted on the body of fluids in the well, and flow of produced fluids to the surface is facilitated. Examples of the gas-lift technique are described in U.S. Pat. Nos. 5,217,067, 4,251,191, and 3,718,407.
U.S. Pat. No. 5,857,519 describes an approach for the downhole disposal of the water component of produced fluids while using gas lift techniques to lift the oil component to the surface. The oil and water components are separated downhole by gravity in an annular space located between a production tubing and the wellbore casing. Pressurized gas is used to drive a downhole pump that re-injects downhole-separated water, and exhaust gas from the downhole pump is used to assist in the lifting of oil to the wellhead.
Using present technology, downhole separation and disposal of water in a wellbore require downhole pumps. The present technology is therefore, inherently plagued with two main problems: 1) complex completions associated with providing power from surface to drive downhole pumps, and 2) poor reliability of the downhole pumps.
What is required is a method and system for the downhole separation and injection of water contained in produced mixtures from hydrocarbon wells that does not require downhole pumps. Accordingly, the present invention concerns a method and system for separating and injecting downhole, the water contained in the produced mixture of a hydrocarbon well while lifting hydrocarbon contained in the produced mixture to surface without the use of a downhole pump.
According to an aspect of the present invention, there is provided a method for the downhole separation and injection of a predominately water component of a production fluid comprising at least some water and at least some oil from a production zone of a hydrocarbon well comprising the steps of separating downhole, at a position elevated with respect to an injection formation, the production fluid into a predominately water component and a predominately hydrocarbon component and delivering the predominately water component to the downhole injection formation, wherein the separating step is conducted at a sufficiently elevated location with respect to the injection formation to permit the predominately water component to be delivered to the downhole injection formation under the force of gravity. In accordance with a preferred embodiment of the invention, the method further comprises injecting gas into the production fluid to deliver the production fluid to the elevated position in the well. In another preferred embodiment, the injected gas is delivered downhole through a gas-lift string that extends from the head of the well.
In accordance with yet another preferred embodiment, the production fluid is delivered to the elevated position by way of a conduit that extends from the production formation to the elevated position. In accordance with yet another preferred embodiment, the production fluid is delivered to the elevated position by way of an annular space within the well.
In a preferred embodiment of the invention, the percentage of water in the production fluid is at least 20%.
In accordance with yet another preferred embodiment of the invention, the production fluid contains gas. In accordance with yet another preferred embodiment, gas is separated from the production fluid and this step optionally precedes the step of separating the production fluid into a predominately water component and a predominately hydrocarbon component. In yet another preferred embodiment of the invention, the separated gas is delivered to the surface.
In accordance with yet another preferred embodiment of the invention, the mostly hydrocarbon component is transported to the surface. In accordance with yet another preferred embodiment of the invention, the separated gas and the predominately hydrocarbon component are combined and delivered to the surface. In a preferred embodiment of the present invention, a mixing device is used to combine gas and the mostly hydrocarbon component of the production fluid.
In accordance with another aspect of the invention, there is provided a system for the downhole separation and injection of a predominately water component of a production fluid comprising at least some water and at least some oil from the production formation of a hydrocarbon well. The system comprises an oil-water separator located downhole at a position elevated with respect to an injection formation, a first passage to provide fluid communication between the production formation and an inlet of the separator, and a second passage to provide fluid communication between the water outlet of the separator and a downhole injection formation. The separator is located at a sufficiently elevated location with respect to the injection formation to permit the mostly water component emerging from the water outlet to be delivered to the downhole injection formation under the force of gravity.
In a preferred embodiment, the oil-water separator comprises at least one cyclone.
In another preferred embodiment of the present invention, the system further comprises means for injecting gas into the production fluid in order to deliver the production fluid to the separator such as a conduit extending between the head of the well and the production formation.
In yet another preferred embodiment, the system includes a gas-liquid separator located at an elevation at least as high as the oil-water separator and having a gas-liquid inlet in fluid communication with the production fluid for receiving the production fluid as well as an outlet for passage of liquid from the gas-liquid separator to the oil-water separator. In a preferred embodiment, the gas-liquid separator comprises at least one cyclone. In another preferred embodiment, the gas-liquid separator comprises at least one auger. In yet another preferred embodiment, the gas-liquid separator comprises a combination of at least one cyclone and at least one auger connected in series or in parallel. In yet another preferred embodiment, the cyclone incorporates a swirl generator.
In yet another preferred embodiment, the system includes a third passage that extends between the oil outlet of the oil-water separator and the head of the well.
In yet another preferred embodiment, the system includes means for injecting gas into the third passage to promote flow of the mostly hydrocarbon component of the production fluid from the oil outlet to the head of the well. Means can include a conduit for providing fluid communication between a gas outlet of the gas-liquid separator and the third passage.
In accordance with yet another aspect of the invention, there is provided a method of completing a well for production of hydrocarbon from an underground formation comprising installing an oil-water separator downhole at a position elevated with respect to the injection formation, providing a first passage for fluid communication between the production formation and an inlet of the separator, providing a second passage that is isolated from the first passage for fluid communication between the water outlet of the separator and the injection formation, and locating the separator at a sufficiently elevated location with respect to the injection formation to permit fluid emerging from the water outlet to be delivered to the downhole injection formation under the force of gravity.
In a preferred embodiment of the present invention, providing an oil-water separator comprises installing at least one cyclone.
In another preferred embodiment of the present invention, the method further comprises providing means for injecting gas into the production fluid in order to deliver the production fluid to the separator. In a preferred embodiment, a conduit extending between the head of the well and the production formation is provided to provide means for injecting gas.
In yet another preferred embodiment, the method further comprises providing a gas-liquid separator located at an elevation at least as high as the oil-water separator and having a gas-liquid inlet in fluid communication with the production fluid for receiving the production fluid as well as an outlet for passage of liquid from the gas-liquid separator to the oil-water separator. In a preferred embodiment, the gas-liquid separator comprises a cyclone. In another preferred embodiment, the gas-liquid separator comprises an auger.
In yet another preferred embodiment, the method further comprises providing a third passage that extends between the oil outlet of the oil-water separator and the head of the well.
In yet another preferred embodiment, the method further comprises providing means for injecting gas into the third passage to promote flow of the mostly hydrocarbon component of the production fluid from the oil outlet to the head of the well. Means include a conduit for providing fluid communication between a gas outlet of the gas-liquid separator and the third passage.
With the present method and system, there does not need to be a downhole pump to inject the downhole-separated water component of produced fluids. The separator is located in a position in the wellbore so as to produce the predominately water component at a sufficient pressure so that it may be injected downhole without the use of a pump. This variable position of the separator can also lead to a reduction in gas-lift requirements. The lower the injection pressure needed to inject the water, the lower the location of the separator which in turn results in reduced artificial lift requirements. Also, with the present system, the produced mixture can be lifted to the separator in either a dedicated tube or annular space. This arrangement leads to a variable tubing configuration for optimizing flow of fluids in the wellbore. Potential benefits include increased production rates in wells currently production limited due to existing tubular and surface facilities, reduction of water handling (both processing and disposal) at the surface, elimination of surface infrastructure for powering downhole pumps, reduced gas-lift usage, reductions in the cost of running high water cut hydrocarbon wells, improved system reliability and environmental benefits from reduced discharge of produced water. As well, gas separated from produced fluids downhole can be commingled and brought to surface with downhole separated oil to reduce tubing requirements in the well.
Other and further advantages and features of this invention will be apparent to those skilled in the art from the following detailed description thereof, taken in conjunction with the accompanying drawings.